There are numerous parameters that are measured invasively and non-invasively to gain diagnostic information about the cardiovascular system. The major function of the cardiovascular system is to supply sufficient amounts of oxygen and nutrients to meet the metabolic demands of the tissues. Cardiac output is a primary determinant of global oxygen transport from the heart to the body and cardiovascular insufficiencies are reflected in cardiac output. Measurement of cardiac output provides both diagnostic and prognostic information, and the means to monitor the adequacy of therapy, as well as to assess the condition of a patient following a major trauma with hemorrhage and during the ensuing fluid resuscitation.
Hemorrhage is a major cause of death of battlefield casualties who are not killed immediately and survive beyond the initial five minutes from injury. Half of battlefield deaths are the result of uncontrolled bleeding. Successful treatment requires immediate medical attention that can be very limited in the battlefield until the casualty is evacuated to higher echelons of care. Monitoring of the physiological status during transport and the end points of the resuscitative treatment is critical to both immediate survival as well as long-term treatment of a patient. It has been suggested that severely injured casualties should be evacuated in less than one hour to a front-line high echelon care unit and that during initial treatment and evacuation, physiological monitoring should be used to guide resuscitation efforts.
Cardiac output is an important indicator of hemodynamic status of the circulatory system and is important in diagnosis and treatment of heart disease and in guiding fluid resuscitation following major trauma with hemorrhage. Cardiac output provides a direct measure of the heart's ability to pump blood and is determined as a product of stroke volume and heart rate. As the hemorrhage progresses, arterial blood pressure decreases and sympathetic compensatory mechanisms are activated that shunt the blood from the peripheral to central compartment in order to maintain blood pressure and thus oxygenation of vital organs. In healthy and physically trained individuals, these compensatory mechanisms maintain blood pressure disproportionately high within the body only to cause a rapid circulatory collapse as the hemorrhage causes massive loss of blood volume. Cardiac output, in turn, reacts faster to the progression of hemorrhagic shock and drops sooner than the arterial pressure in response to blood loss during hemorrhage. Early intervention and resuscitation based on monitoring of cardiac output is the most complete way of capturing the physiological impact of the hemorrhage and resulting circulatory shock. In addition to the delay in blood pressure drop caused by the compensatory mechanisms, arterial blood pressure is not available under battlefield conditions and is commonly substituted by traditional, non-invasive (occlusive) blood pressure measurements in peripheral circulation. As mentioned above, the compensatory mechanisms that maintain blood supply to the vital organs do so by increasing peripheral resistance and diverting blood from the peripheral to central pool, causing the cessation of pressure pulse in peripheral circulation, and further difficulties in the determination of blood pressure. Although blood pressure is currently used to monitor the progression of hemorrhagic shock and the effectiveness of fluid resuscitation, direct measurement of cardiac output would provide a more effective, direct measure of both and, consequently, a more effective therapy.
Clinicians have an increasing number of available sensors and instrumentation that support measurements of cardiac output. These include, without limitation, indicator dilution techniques with or without the use of a pulmonary artery catheter, arterial pulse contour techniques, aortic pulsed Doppler, both of the ascending and descending aortas, indirect measures using arterio-venous gas content differences and expired gas measures via the Fick's equation, and bio-impedance techniques. Due to the high costs and need for highly specialized medical personnel and well-equipped facilities, as well as potentially severe complications associated with invasive measurement of cardiac output, noninvasive techniques are highly desirable. For battlefield applications, only non-invasive methods that require minimal skills and that can be deployed under “far forward” and “casualty transport” conditions are practical. None of the above methods has as yet fulfilled these requirements.